1. Field of the Invention
The present invention relates to a fixed-focus imaging lens which is suitable for use in small imaging apparatus as exemplified by digital cameras using an imaging device such as a CCD (charge-coupled device) or a CMOS (complementary metal oxide semiconductor) sensor and cameras using a silver salt film.
The invention also relates to a camera module which converts an optical image formed by the above imaging lens into an imaging signal and to a portable terminal apparatus such as a cell phone with a camera or a personal digital assistant (PDA) which incorporates the above imaging lens and performs shooting.
2. Description of the Related Art
In recent years, with the spread of personal computers to homes etc., digital still cameras (hereinafter referred to simply as digital cameras) capable of inputting image information obtained by shooting a scene, a person, or the like to a personal computer have spread rapidly. Furthermore, many cell phones have come to incorporate a module camera for image input (cell phone module camera).
Such imaging apparatus employ an imaging device such as a CCD or a CMOS sensor. In recent years, these imaging devices have become more compact and increased compactness has come to be required also for the entire imaging apparatus and an imaging lens mounted therein. At the same time, the number of pixels of imaging devices has been increased and imaging lenses have come to be required to be increased in resolution and performance.
In the above circumstances, JP-A-2004-302058, JP-A-2005-173319, JP-A-2005-227755 and JP-A-2005-292235 disclose imaging lenses which employ a three-lens structure and aspheric lens surfaces and are thereby increased in compactness and performance.
As described above, recent imaging devices have been reduced in size and increased in the number of pixels and, accordingly, a high resolution an increased compactness have come to be required for imaging lenses for digital cameras. On the other hand, recently, imaging lenses of cell phone module cameras, which were mainly required to be compact and of a low cost in the past, have also come to be required to be increased in performance with a tendency of increase in the number of pixels of imaging devices.
As a result, it is now desired to develop a wide variety of lenses that are improved in a total sense in terms of image forming performance and compactness. For example, it is now desired to develop high-performance imaging lenses which are compact enough to be incorporated in cell phone module cameras whereas exhibiting sufficiently high performance to be incorporated in digital cameras.
One method for satisfying the above requirements is to employ a three-lens structure for the purpose of increase in compactness and positively use aspheric surfaces for the purpose of increase in performance. Although the lenses disclosed in the above Patent documents employ a three-lens structure and aspheric surfaces, they are still insufficient in image forming performance and compactness, for example.